1. The uptake of 28Mg, and the total tissue content of Mg, Ca, Na and K have been determined in the smooth muscle of the guinea‐pig taenia coli. The Mg content was 6·56 m‐mole/kg fresh wt. immediately after dissection, falling slowly to 5·11 after 6 hr immersion in Krebs solution at 37° C. 2. The Mg content rose to 15·4 m‐mole/kg fresh wt. during immersion in isotonic sucrose containing only MgCl2. It was independent of the Mg2+ concentration in this solution, but was depressed when K+ or Ca2+ ions were added. 3. 28Mg uptake showed three separate phases, extracellular, intermediate and slow. The size of the extracellular phase was proportional to the Mg2+ concentration in the solution, but the size of the slow phase was constant. The size of the intermediate phase, exchanging with a half‐time of a few minutes, was depressed when K+ or Ca2+ ions were added. 4. The results are compatible with a competition between Mg in the intermediate phase of tracer exchange, and K+ or Ca2+ ions for fixed anionic sites in the tissue. © 1969 The Physiological Society
